In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Ceramics such as zirconia-based materials have emerged as high-strength framework materials for dental prostheses (single-units up to multiple unit bridges). However, such materials are often an intense white color, and consequently the esthetics of the finished restoration is unacceptable in its natural color.
A finished dental restoration should match the color of the patient's teeth, i.e., it should be “tooth colored”. The colors of human teeth appear to range from a light almost white-tan to a light brown, and occupy a very specific color space. This color space can be described by the commonly used CIE (Commission Internationale de l'Eclariage) L*, a*, b* conventions, which represents colors in a three-dimensional Cartesian coordinate system. L*, or “value,” is a measure of luminance or lightness, and is represented on the vertical axis. The a*, b* coordinates, are a measure of chromaticity and are represented on horizontal coordinates, with positive a* representing red, a negative a* representing green, a positive b* representing yellow, and negative b* representing blue. U.S. Pat. No. 6,030,209, which is incorporated herein by reference, presents the CIE L*, a*, b* color coordinates of tooth colors represented by the Vita Lumen® shade guide system manufactured by Vita Zahnfabrik (i.e., it presents the color space of tooth colors). As used herein, “tooth color” or “tooth-like color” is taken to mean CIE L*, a*, b* color coordinates that fall within, or very close to, this color space.
Currently there are two main commercially available methods to deal with the stark white color of dental ceramics such as zirconia. In the one method, the white ceramic is “hidden” by applying either a layer of stain or a liner to a sintered framework. The other method entails shading the ceramic by immersion in, or painting with, coloring solutions while in the pre-sintered state. Coloring with stain/liner is undesirable as it is an extra fabrication step and because it lowers translucency. Shading with a coloring solution is undesirable because it requires the extra step of dipping or painting, and extra time to dry before sintering.
U.S. Pat. No. 6,713,421 appears to describe pre-sintered zirconia blanks that are colored with 0.2-1.5 wt. % coloring additives, selected from the group consisting of the oxides Fe2O3, Er2O3 and MnO2. The blanks are intended for CAD/CAM processing into dentures. French patent publication 2,787,366 and Cales et. al. (“Colored Zirconia Ceramics for Dental Applications,” Bioceramics Vol. 11, edited by R. Z. LeGeros and J. R. LeGeros; Proceedings of the 11th International Symposium on Ceramics in Medicine; York, N.Y.; November 1998) appear to identify a number of colorants, and was reportedly successful in achieving some of the Vita shades by using combinations of Fe2O3, CeO2 and Bi2O3. Bushan et al. (S. Bushan, R. Pober and R. Giordano, “Coloration of Partially Stabilized Zirconia,” Abstract 1775, Journal of Dental Research, Vol. 84, Special Issue A, 2005) appears to describe the study of the coloration of partially stabilized zirconia by a variety of solutions comprising coloring cations. U.S. Pat. No. 5,219,805 appears to disclose coloration of stabilized zirconia for dental bracket applications using combinations of Fe2O3, Er2O3, and Pr6O11. U.S. Pat. No. 5,656,564 appears to describe coloration of zirconia with combinations of Er2O3 and Pr6O11. U.S. Pat. No. 5,011,403 appears to describe coloration of zirconia using combinations of Fe, Ni and Mn in the form of water solutions of sulfates and acetates, respectively, added to a ceramic slip. U.S. Pat. No. 6,709,694 appears to describe the use solutions for coloring of pre-sintered zirconia by immersion, painting or spraying using a metal ion coloring solution or metal complex coloring solution that is applied to a presintered ceramic, followed by sintering to form a translucent, colored dental ceramic. The ions or complexes are of the rare earths elements or subgroups II and VIII, with an action time of less than two hours, and maximum pre-sintered zirconia diameter and height of 10 and 7 mm, respectively. All of the above identified documents are incorporated by reference herein in their entirety
The coloring of technical zirconias is also documented. For example, the development of pink coloration in zirconia by Er additions is described in: (i) P. Duran, P. Recio, J. R. Jurado, C. Pascual and C. Moure, “Preparation, Sintering, and Properties of Translucent Er2O3-Doped Tetragonal Zirconia,” J. Am. Ceram. Soc., vol. 72, no. 11, pp. 2088-93, 1989; and (ii) M. Yashima, T. Nagotome, T. Noma, N. Ishizawa, Y. Suzuki and M. Yoshimura, “Effect of Dopant Species on Tetragonal (t′)-to-Monoclinic Phase Transformation of Arc-Melted ZrO2—RO1.5 (R═Sm, Y, Er, and Sc) in Water at 200° C. and 100 MPa Pressure,” J. Am. Ceram. Soc., no. 78, no. 8, pp. 2229-93, 1989. Additions of CoO, Fe2O3 and Cr2O3 combinations to yttria-stabilized zirconia are known to impart a blue color in the final sintered zirconia bodies, as apparently described in Japanese patent publication 2,145,475. Additions of NiO and Cr2O3 to yttria-stabilized zirconia have been shown to result in a purplish colored sintered body, as apparently described in Japanese patent publication 5,043,316. All of the above identified documents are incorporated by reference herein in their entirety.
Most of the aforementioned coloring additions can negatively affect not only mechanical properties, including strength and fracture toughness, but also isotropic shrinkage and final sintered density. This can happen for a number of reasons including: (1) loss of fracture toughness from a lowering of the “transformation toughening” effect as a result of the over-stabilization of the tetragonal phase by the additive (either chemically, or by grain size reduction) thereby hindering the transformation from the metastable tetragonal phase to monoclinic phase that is necessary for the toughening to happen; (2) loss of strength due to spontaneous microcrack formation that can result if grains grow too large because of the coloring additive; and (3) loss of strength due to the formation of strength-limiting pores in the microstructure due to the coloring additive. This last reason is what Shah et al. (K. C. Shah, I. Denry and J. A. Holloway, “Physical Properties of Cerium-Doped Tetragonal Zirconia,” Abstract 0080, Journal of Dental Research, Vol. 85, Special Issue A, 2006) attribute the significant loss of strength, down to 275±67 MPa, for 3YTZP materials that were colored using Ce salts. The problem of formation of larger pores, along with grain growth, in colored zirconia sintered compacts has also been recently recognized by Omichi and Takei (N. Omichi and T. Takei, “Colored Zirconia Sintered Compact and its Production Process,” JP 2005289721, Oct. 20, 2005).
Thus, it would be beneficial to provide ceramic materials with a desired coloring without resorting to extra processing steps such as required for the staining/liner or coloring by solutions (liquids) techniques, and without any significant compromise of physical properties of the resulting densified ceramic material.
Often times, the color of an as-sintered ceramic materials may exhibit instability when subjected to certain conditions or environments. For example, the color of as-sintered zirconia frameworks may not be stable when subjected to the subsequent firings which are necessary to overlay the framework with porcelain to produce a finished restoration. Typically, an overlay porcelain for zirconia frameworks is fired, or pressed, under vacuum at levels of approximately 35 torr at temperatures of 750-1065° C. These conditions are reducing relative to the atmosphere in which the zirconia is sintered. Since the color of zirconia can be affected by exposure to reducing conditions as illustrated, for example, by Romer et al. (H. Romer, K.-D. Luther and W. Asmus, “Colored Zirconia,” Cryst. Res. Technol., 29, 6, 787-794 (1994)), the color of the framework may not be stable. For example, Romer et al. documented the following color transitions for 12 mol % yttria stabilized zirconia crystals colored with various dopants upon exposure to oxidizing versus reducing conditions at 1100° C.: yellow to red-brown for vanadium, green to brown for chromium, violet to pale orange for manganese, green to yellow for iron, violet to violet-blue for cobalt, and colorless to yellow for nickel. It is understood that the underlying reason for the change in color is the change in the thermodynamic oxygen vacancy concentration of the zirconia, as dictated in part by the oxygen potential of the firing atmosphere. Due to charge compensation reasons, this can effect a change in the valence of the coloring dopant, and thus change the color of the zirconia. The effect can be especially rapid in zirconia due to an inherently high oxygen diffusivity. During the porcelain overlaying steps the inside of a restoration is exposed, it is thus susceptible to changes in color due to the vacuum firing conditions. Additionally, the oxygen potential of the porcelain, which is expected to be different than that of air, can also contribute to color change for the same reason. It is speculated that the change in the zirconia oxygen vacancy concentration effects a change in color due to a change of the valence of the chromophore ion, for charge compensation, and/or due to a change in the number or oxygen ions coordinated with the chromophore ion.
Thus, it would be extremely beneficial to provide ceramic materials with a coloration that is stable when subjected to certain conditions or environments, such as color stability of dental articles during subsequent processing steps required to produce finished dental restorations.
The propensity for ceramics such as zirconia to become colored by the addition of relatively small amounts of coloring elements can lead to accidental discoloration due to unintended exposure to impurities in the furnace chamber during sintering. The impurities can come from a variety of sources, including the heating elements and kiln furniture, and can build up in the furnace over time. For example, zirconia frameworks will periodically emerge from sintering with yellow discoloration, a problem that is associated with corrosion of molybdenum disilicide heating elements. Consequently, the frameworks, which are supposed to be white, often must be discarded. A solution for this problem has been to run the furnace at a high temperature under good ventilation and with an empty firing sagger several times, which has the effect of regenerating the characteristic protective glassy silica layer on the heating elements, and eliminating the heating element corrosion. It is speculated that the corroded heating elements are the source of an impurity (or impurities), that has a significant enough vapor pressure during the sintering cycle to effect its transport into the zirconia framework and cause yellow discoloration. Although the regeneration of the protective silica layer on the heating elements effectively seals the source of the offending discoloring impurity, and presumably eliminates and or neutralizes it in the furnace lining and furniture thereby permitting for the subsequent sintering of non-discolored frameworks, the operator still unfortunately has to sacrifice some frameworks to get to this point, costing time and money. Regardless of the source of the discoloring impurity or how it gets into the zirconia, it would be useful to have a method to avoid such discoloration.
Thus, it would be would be beneficial to provide a method for sintering ceramic materials, such as white zirconia frameworks, that does not discolor the material with impurities introduced by components or otherwise present in the sintering environment.
Yttria-stabilized zirconia can have lower than desirable hydrolytic resistance, or rather, resistance to hydrolytic degradation, also known as hydrothermal aging or low temperature degradation. Hydrolytic degradation is characterized by the spontaneous transformation of the tetragonal phase into monoclinic upon exposure to water for extended time. It typically occurs at the surface of a densely sintered body after long exposures in warm aqueous environments, and can debit mechanical properties, and is hence undesirable for dental applications. It well known that the addition of aluminum in amounts of 0.1-0.4 wt. % (oxide form, Al2O3) to 3 mol % yttria-stabilized zirconia (3YSZ) will increase hydrolytic resistance. Although there are several commercial alumina doped 3YSZ powders available they are usually more expensive than their alumina-free counterparts. Also, it is possible that the hydrolytic stability of the available alumina doped 3YSZ ceramics can be improved by increasing the alumina content.
Thus, it would be beneficial to provide a method for sintering a ceramic body in a manner that introduces aluminum into the ceramic during sintering.